1. Field of the Invention
The present invention relates to a circuit for quantizing image data used for representing analog image data in the form of a digital quantization code having a predetermined number of bits.
2. Description of the Related Art
In general, a method for effecting A/D conversion on analog image data has heretofore been carried out in the following manner. That is, the range of values corresponding to the differences between the maximum values and the minimum values both selectable by respective pixels of an original image is divided according to the number of bits capable of being taken out as an output. Each of divided level ranges corresponding to the taken-out number of bits is represented in the form of a code using the corresponding number of bits. It is next decided to which level range the level of each pixel belongs. Further, a code indicative of the divided level range to which the level of each pixel belongs, is taken out as a digital output signal. In this case, as a decode level (decode representative level) of the code which represents each divided level range at the time that an D/A converting process is effected, a central value of each level range is used.
This method is equivalent to a method of minimizing quantization distortion developed in each pixel unit. When the level division is uniformly carried out, it is subjected to linear quantization. On the other hand, when the level division is uniformly made depending on the probability of development of a pixel level, it is subjected to nonlinear quantization. However, the norm of minimizing the quantization distortion developed in each pixel unit is applied even to either case as it is.
Since the above-described A/D converting method can minimize a quantization error developed for each pixel and provide a satisfactory S/N ratio, the analog level of the original pixel can be replaced with a decode representative level nearest to its analog level.
However, the A/D converting method has a problem in that although it is quantitatively satisfactory, visually remarkable deterioration is often developed in the restored or reproduced image. That is, this occurs because a human being sees or takes an image as a time variation (motion) in image or a spatial variation (details such as resolution, an outline or contour of an image, etc.) in image without taking the image as one pixel unit.
When such a variation that the level of an image signal abruptly rises and reaches a flat level occurs as indicated by a solid line in FIG. 6, for example, a decode signal at the time that the conventional A/D conversion for minimizing the quantization distortion for each pixel is effected is represented as indicated by a solid line b (corresponding to a line formed by connecting points indicated by black circles or dots to one another). Even in the case of a range FW indicative of the flat level, the signal is brought into a "jagged or notched" state and is not faithful to a visual variation.
Such notches give rise to deterioration in image when the reproduced image is seen with human eyes. When the notches are developed in a time direction, they are brought to so-called jerkiness. A so-called edge busyness appears at an edge portion extending in a spatial direction, whereas so-called mosquito noise is produced at a flat portion.
Thus, the conventional A/D converting apparatus has been designed uniformly based on the standard for minimizing the quantization distortion for each pixel and has been out of accord with a human visual characteristic. The more the number of bits increases, the more visual deterioration in image is made nonprominent. However, the conventional A/D converting apparatus has a problem in that since the quantized number of bits corresponding to an output signal is fixed, an increase in the quantized number of bits runs to waste and the efficiency of transmission is reduced in the case of such an image that the visual deterioration is not developed.
Even in the case of images which are in accord with the human visual characteristic, they are different in desired or favorite image quality from each other according to differences among individuals. Since the conventional A/D converting method can simply obtain the uniform image created under the standard for minimizing the quantization distortion for each pixel, as each reproduced or restored image, only a method of effecting adjustments such as contrast, tints, sharpness, etc. on a reproduced analog image signal was used to set each image to the desired quality of image.